Various methods are known for extracorporeal blood treatment, in which the blood of the patient flows in an extracorporeal blood circuit through a blood treatment unit. The possibility of penetration of air into the extracorporeal blood circuit represents one of the fundamental complications of extracorporeal blood treatment, for example, haemodialysis or haemofiltration.
The known drip chambers, which are positioned in the venous branch of the extracorporeal blood circuit downstream of the blood treatment unit, are used for separation of entrained air bubbles from the blood. The known drip chambers intercept the air bubbles with a high degree of certainty. Nevertheless, there is basically the risk that air bubbles are infused into the patient intravenously. Therefore, for further increase of safety, apparatuses are provided in blood treatment devices with which the occurrence of air bubbles can be detected.
Apparatuses for detection of air bubbles and blood are described, for example, in German Application Nos. DE 102 09 254, DE 10 2005 025 500 and DE 2005 025 515. The known air detectors are based on the difference of absorption of ultrasound in liquid and gaseous media as well as the scattering of ultrasound at interfaces. For detection of air, signal pulses are coupled into the blood, while the signal pulses exiting from the blood are received. It can be concluded from this that air is present when the received signal falls below a reference value. Apart from ultrasonic sensors, air detectors are also known that are based on the different dielectric constants and conductivities of liquid and gaseous media.
The formation of smaller or larger air bubbles in blood can have different causes. Possible causes for the penetration of air are a leak in the tube line system, a fall in the blood level in the venous drip chamber or a single entry of air during an infusion of a drug into the extracorporeal blood circuit. However, microbubbles, which derive from cavitation, can also occur in the blood.
In general, occluding blood pumps are used for conveying blood, in particular roller pumps, which have several rollers with which the tube line is occluded, as a result of which a pulsating blood flow is produced. On the input side, the pump produces a negative pressure in the blood line and, on the outlet side, a positive pressure. The negative pressure on the suction side of the pump is due to a constriction of the flow cross-section in the cannula for connection to the patient, while the outlet side positive pressure is determined by the resistance to flow in the tube line system.
In the tube line upstream of the blood pump, cavitation can lead to the formation of microbubbles. The number of microbubbles typically increases from the arterial cannula to the arterial blood line and to the outlet of the blood pump. The marked increase at the outlet of the pump can be explained by the especially strong cavitation as the roller of the roller pump lifts, with a brief minimum clearance between roller and tube, and a high pressure gradient between pump inlet and outlet. The microbubbles, once created, are exposed to a positive pressure, downstream of the pump in the tube line system, due to which they can continually be dissolved again. Furthermore, in the blood treatment unit, in particular in the dialysis machine, the number of microbubbles is reduced by exchange for degassed dialysis fluid, as a result of dissolved air from the bloodstream passing via the dialysis machine into the degassed dialysate. Thus, the re-solubility of the blood increases, because of which the number of microbubbles in the blood sharply decreases.
A certain number of cavitation-related microbubbles can be re-dissolved in the tube line system and the dialysis machine insofar as they are no longer detected by the devices with which the number of air bubbles occurring in a predetermined time interval is compared with a threshold value. However, intensified cavitation leads to a larger number of air bubbles, which is detected by the monitoring devices. If a specific threshold value is exceeded during the treatment, the monitoring device triggers an alarm. Since large quantities of microbubbles are in the tube section between the venous drip chamber and the venous cannula at the time when the threshold value is exceeded, the blood treatment must be stopped for separation of the microbubbles. After dialysis has been stopped, the patient must be separated from the arterial and venous blood line and the ends of the tubes have to be short-circuited with a special adapter, in order to pump the blood to the extracorporeal blood circuit. While the blood slowly recirculates in the closed circuit, the microbubbles can be dissolved or can be separated in the venous drip chamber. The patient must then be re-connected to the extracorporeal blood circuit.
By monitoring the extracorporeal blood circuit, the occurrence of microbubbles is detected, an alarm is triggered, and the medical personnel are prompted to initiate the necessary measures to eliminate the infusion of microbubbles. However, there is a disadvantage that interruption of treatment is connected with increased effort for the medical personnel and inconvenience for the patient.
The measures to be initiated when cavitation-related air bubbles appear are different from the measures that are necessary when microbubbles occur due to micro-leaks in the tube line system or the lowering of the blood level in the drip chamber.